This invention relates to an internal combustion engine, in particular of the self-igniting type, with a compressor, which is preferably connected to an exhaust gas turbine and is located in an intake flow passage for compression of the intake air, and with a charge air line leading to an intake manifold, the compressor operating independently of the engine, at least in the starting phase.
By reducing the compression ratio the weight of the engine may be reduced, especially with diesel engines, since a lightweight crankshaft and smaller crankshaft bearings may be used. With internal combustion engines having a low compression ratio in the cylinder the low final compression temperature resulting therefrom will not suffice during a starting operation at low external temperatures to reach the temperature required for self-ignition of the diesel-air mixture. By preheating the combustion air before and during starting this disadvantage may be compensated.
For the purpose of preheating the combustion air several measures to assist starting may be employed. On the one hand the required fuel ignition temperature may be produced locally in the combustion chamber with the use of a heat source, such as a glow plug, or the intake air may be preheated, for example by means of a flame starter, or electrical energy may be introduced into a heater flange. Furthermore, volatile ignition accelerators or improvers may be introduced in liquid or gaseous form, which, apart from the necessary additional metering devices, has the disadvantage of necessitating yet another medium.
The main argument against a heater flange is that at temperatures below zero degrees centigrade the heating process may well take longer than sixty seconds. This is not only time-consuming but will also cost considerable energy, as it requires the heating of both the air and the heat exchange faces, which consist of metal and have a much higher heat capacity than air. The heat exchange faces must have a close mesh to ensure good heat transfer from heater flange to air, which will however impede the air flow during operation of the engine. Glow plugs require additional space in the cylinder head, which is not always available.
An alternative for supercharged diesel engines would be to operate the compressor for the charge air by means of an additional drive during starting and to preheat the cylinders by compression of the intake air. U.S. Pat. No. 5,704,323 describes an internal combustion engine comprising a turbocharger which is electrically driven during start-up in order to increase the temperature of the intake air. The air taken in at ambient temperature is compressed and forced into the cylinders directly via the intake valves. At a low compression ratio and low ambient temperatures a relatively long time will be required for preheating the combustion air with this method.
In DE 37 21 522 A1 and GB 2 163 483 A a technique is described where the output side of the compressor is connected to a valve-controlled air recirculation line in order to control the boost pressure of the compressor. DE 14 51 898 A describes an internal combustion engine with an exhaust gas turbine, where downstream of the compressor an automatic controller is located in the charge air line, which connects the output side of the compressor to atmospheric pressure during starting, in order to reduce the start-up phase of the charge blower. With these known devices the compressor cannot be operated independently of the internal combustion engine during starting, and the time required for heating up the combustion air at low ambient temperatures will not be significantly reduced.
It is an object of the invention to avoid these disadvantages and to improve the starting properties of internal combustion engines, particularly of self-igniting engines with low compression ratio.
According to the invention this object is achieved by providing that a valve-controlled air recirculation line branch off from the intake manifold, which opens into the intake flow passage upstream of the compressor, charge air line and air recirculation line forming a closed loop. The air prevailing at ambient pressure and temperature is compressed in the compressor. The air leaves the intake manifold through the air recirculation line and is decompressed to ambient pressure and returned to the suction side of the compressor, which is driven electrically, hydraulically or pneumatically during the starting phase.
Via a throttle in the air recirculation line the pressure of the air from the intake manifold is relieved to ambient pressure level. The throttle will not be required if the diameter and surface roughness of the air recirculation line are chosen in such a way that the same throttle effect will be obtained.
Thermodynamically, the heating of the air is effected in two steps. In the first step the compressor transforms electrical energy into compression energy. In the second step the compression energy is transformed in the throttle into increased flow velocity and, partially, into frictional heat. The compression/decompression cycle may be repeated as often as desired. Besides, the air recirculation process will ensure that hot air will reach all parts of the intake manifold.
If an intercooler is provided in the charge air line, the invention provides in a further step that the intercooler be bypassed via a bypass line. This will prevent the charge air and thus the intake manifold from cooling off.
To prevent the admission of cold air into the cylinders it is of advantage if the intake valves of the internal combustion engine are kept closed during starting. Closing of the intake valves in the start-up phase may be effected by a variable valve actuation device. If a variable valve actuation device is not possible the time for heating the combustion air will be prolonged.
During starting of the compressor a vacuum is generated in front of it. For compensation of this vacuum it is provided that the recirculation system be subject to ambient pressure at the suction side of the compressor, at least at the beginning of the starting phase. Due to the air entering the recirculation system the vacuum in front of the compressor can be compensated.